Method of prospecting for buried deposits



Jan; 16, 1945.

E. McDERMoTjr METHOD OF PROSPECTING FOR BURIED DEPOSITS Filed Jan. 22, 1940 2 NQ@ @SRA Patented Jan. 16, 1945 UNITED STATES PATENT oFFlcE METHOD F PROSPECTING FOR BURIED DEPOSITS Eugene McDermott, Dallas, Tex.

Application January 22, 1940, Serial No. 315,060

i Claims.

This invention relates to a method of prospecting for deposits such as oil or gas. This application is a continuation-in-part of my applications Serial No. 261,303, filed March 11, 1939; Serial No. 258,774, filed February 27, 1939; Serial No. 255,559, led February 9, 1939; and Serial No. 257,329, led February 20, 1939.

The invention is based on the discovery that high concentrations of various mineral deposits exist in the earth near its surface over buried accumulations of oil and gas. i

It is well known that the sub-surface waters of the earth contain very appreciable quantities of minerals in solution. I have found that the concentration of such minerals near the surface of the earth bears a. significant relation to the location and size of buried oil and gas deposits. By determining the quantity of such minerals present in the earth near the surface and plotting the relationship of such determinations, it is possible to located buried oil or gas deposits.

The mineral concentration near the surface is caused in part vby migration from greater depths. The gases leaking to the surface will carry with them some water which is present in the sub-structure of the earth. In passing through a water rich in minerals appreciable quantities of these will be carried to the surface of the earth. En route some of the minerals will be deposited. Some of the minerals thus carried toward the surface of the earth are dissolved in the water, while others are held in colloidal suspension in the Water. Where the rate of leakage from the buried deposit is greatest, the concentration of minerals at or near the surface will also be greatest. K

Because ofthe fact that heavier hydrocarbons accumulate in the cap rock, usually shale, above the oil or gas deposit, the cap rock becomes less permeable vto the passage of the hydrocarbon gases. This is so especially wherechlorine is present as it tends to oxidize and polymerize the hydrocarbon gases which leak from the deposit and migrate to the surface and thereby produces liquid and solid hydrocarbons which fill the pores of the rocks just above the oil or gas deposit and render such rocks impervious. This causes the migration to move to the edge of such impervious section and then upwardly. The resultant effect is that the concentration of minerals near the surface-directly above the oil or gas deposit may be normal While around the y tral point spaced inwardly from the inner edges of the band pattern.

In some cases, however, the greatest ocncentration at the surface will be directly over the oil or gas deposit. This situation occurs, for example, when folds in rock shale have caused fissures over the top of the accumulation. However, this type of pattern is the exception as in the vast majority of cases the high concentrations occur over the edges of the buried accumulation.. y

It is, of course, true that there will be small variations in the concentrations at the surface in a given locality which may result from recent leaching such as may occur from drainage because of the'particular topography of the area being tested. However, much larger v ariations have been found by quantitative measurements in many areas to be due to the presence of buried petroleum deposits and, as a rule, the small variations due to leaching are negligible as compared to the larger variations resulting from the presence of buried petroleum deposits.

I have discovered that by'systematically taking a plurality of samples of the earth near the surface at predetermined locations andthen analyzing each sample for its contents in cer` tain inorganic values, and then plotting the results, either in prole or plan, or both, it is possible to determine the probable location of a buried deposit. Such a procedure in many instances will disclose a band pattern of a high concentration area, as mentioned above, or occasionally a high concentration area directly above the oil or gas deposit, as in the case of Aa ssure directly above such accumulation.

Generally speaking, I analyze the soil samples for their water soluble mineral content, after `which I plot the results of such analyses. Where the results of the plotting, either in plan or in proiile, or both, show a band pattern of high concentration surrounding an area of low concentration, or a high concentration area bounded by areasof low concentration, it is highly probable that an oil or gas deposit can be located. In the first instance the oil or gas deposit should be located in the area of low concentration which is surrounded by the band pattern of `high concentration. In the second instance the oil or gas deposit is probably directly beneath the area of high concentration.

In accordance with the invention I have devised a. method which simpliesA the procedure materially since, in accordance therewith, it is not necessary to measure all of such mineral contents separately.

Specifically, I may analyze the soil samples for one or more of the following:

(a) The soluble mineral content. (b) The ionizable mineral content. This is one type of analysis for soluble mineral values.

(c) The sulphate content.

(d) The chlorine content.

(e) The mineral content which is not soluble but the particles of which will enter into a colloidal suspension.

I may also analyze for the combustible carbon content, but such latter method will not be described herein as it constitutes the subject matter of a companion application Serial No. 257,328, led February 20, 1939.

After the determinations of the soil samples have been made' for the respective contents mentioned, the results are plotted in profile or in plan, or both, in the manner recited and which will be more apparent hereinafter from a study of the drawing figures and the description thereof.

In some areas it is desirable to measure the total soluble mineral content ofthe sample. In others it has been found more satisfactory to measure the ionizable minerals. In still others it is desirable to measure a specific anion such as a sulphate or chlorine. Depending on local conditions, it may be advisable and desirable to make all five measurements, and then to plot them al1 as mentioned.

The main objects of my invention ,are manifest from the recitation of purpose and method given above. Other objects of the invention shall be more apparent from the following specific description and the appended claims when read in conjunction with the accompanying drawing, in which:

Fig. l is a diagrammatic representation of an area which has been plotted in proiile;

Fig. 2 is a diagrammatic representation of an area which has been plotted in plan;

Fig. 3 is a diagrammatic showing of an apparatus for testing soil samples for the ionizable mineral content.

The representations of Figs. 1 and 2 are typical of actual operations carried out in accordance with the invention and indicate typcal examples of the results of testing soil samples for the various contents mentioned hereinbefore.

In Fig.'1, the location of the samples is shown horizontally Where the interval between sampling points may be one-eighth mile. In some cases it may be desirable to use a smaller or larger interval. The vertical scale in the figure indicates the concentrations of minerals in one-hundredths of a per cent by Weight of soil samples. High concentrations are shown to occur at two spaced intervals. The proper location of a well to test the results would be between these two high concentration areas.

In Fig. 2, the results of a survey are shown in plan. 'I'he numbers represent concentrations in one-hundredths of a per cent by weight of sample. The area oi.' high concentration shown in Fig. 2 forms a band pattern of the type discussed above. In accordance with the theory of the invention, and as verified by actual surveys made in accordance with the invention, since the area of high concentration shown in Fig. 2 will occur over the edges of the buried deposit, it is obvious that the proper location for a test Well -Should be in the low'concentration area which is inside the high concentration area.

In the area of high concentration, values of greater than one-tenth per cent are frequently found. whereas outside of this area values of a few hundredths of a per cent may exist. The ratio of the values in the high concentration area to those outside of it is very appreciable.

Such patterns as have been mentioned have been found to exist, for example, at the East Texas oil field and at the La Rosa field in Refugio County, Texas. In the band of high values above the edges of the buried deposits values of the order of 1,600 parts per million were found, whereas, elsewhere, including the area over the iield, values of the order to 200 parts per million prevailed. Samples were taken from a depth of approximately ten feet below the earths surface.

In the case of ionizable measurements the figures given in the drawing would not be as given therein, but figures would be substituted to indicate conductivity in michroms. This will show a pattern similar to those referred to above.

In the case of colloidal minerals the same type of patterns results but the values are expressed in terms of relative adsorption, as will be more apparent hereinafter. Frequently these values will be ten times higher in the band patterns than outside such patterns.

For a reconnaissance survey samples may be taken at approximately one-eighth mile intervals along approximately parallel lines about one mile apart. For detail work lines one-half mile apart will generally suiiice. It is notrequisite, of course, to take the samples at regular intervals. In general for reconnaissance surveys a density of four points per square mile and for detail surveys a density of eight points per square mile will be suiiicient.l

40 By soil sample is meant any sample of the earth taken near its surface. It has been found a depth of ten feet is quite satisfactory, though samples may be taken a smaller or greater depth. The exact depth of sampling is not critical, but where because of the particular typography drainage has been heavy, or because of excessive weathering or leaching, the samples should be taken a sufficient distance below the surface to obtain soil not materially affected by such conditions.

For all the determinations of sample contents, the sample is prepared by iirst drying at a temperature of about 50 C. for a matter of several days until the sampleis water free. The sample is then thoroughly ground and mixed and a portion of approximately 10 grams (though more or less may be used) is weighed out for the purpose of making the determination.

Having now described the method generally, specific disclosures oi' the manner of testing for (a) sulphates, (b) chlorine content, (c) water soluble minerals, (d) ionizable minerals, and (e) colloidal minerals, will now be given.

Sulphates A method of determining the sulphates in accordance with the present invention is as follows:

Stir 50 grams of soil (previously dried at 50 C. in oven and ground to pass No. 60 mesh) with 500 cc. of distilled water for five minutes and lter.

Evaporate 250 cc. of the filtrate to dryness, add 40l cc. of 12 molar HCl, evaporate until solution reaches syrupy consistency; add 3 cc. l2 molar HC1 and follow with 40 cc. of hot water. This solution is filtered through a paper filter and washed with 0.1 molar HC1 keeping the final volume of the filtrate to 100 cc.

To th'e cold illtrate add 10 cc. of 0.25 molar. BaClz and set aside for forty-eight hours, then check to see if all of the sulphate has been precipitated by adding a few more drops of BaClz to the clear solution above the precipitate. The addition should be continued until the barium ion is in excess. The BaSOi precipitate is filtered on a filter paper and washed with a hot solution containing cc. of l2 molar HC1 and 1 gram BaC12.2H per liter until washings give no test for iron with potassium thiocyanate, then wash with hot water until washings give no test for chlorides with AgNOs. The paper and precipitate is ignited in a weighed crucible, one or two drops of 6 molar H2804 acid added to convert any BaS to BaSO4. Ignite to drive off the excess H2804, cool and weigh. Repeat treatment with H2804, ignite again, and weigh. Repeat this process until weighings agree within 0.1i. mg. A blank is run at the same time as the sample and the results corrected accordingly. From the resuits the sulphate present is computed.

Other methods may be used to determine the sulphate content. v

Chlorine content There are several methods of determining the chlorine present in the soil in the form oi chlorides. One method which has been found very satisfactory is to mix the soll with water and a small quantity of potassium chromate to serve as an indicator and titrate with silver nitrate solution. The quantities of silver nitrate necessary to obtain a color change is a measure of the chlorine content. One variation of lthis method is to mix the soil with water, llter, and then treat the water in which the chlorine is dissolved as described above.

Soluble minerals As previously stated, the sub-surface water of the earth contains very appreciable quantities of minerals in solution, and as a result high concentrations of these minerals may be found to exist in the soil near the surface of the earth over a buried oil or gas accumulation. Of course, it is possible to determine the mineral content by analyzing for the different minerals separately. This, however, is a tedious and difficult process for some of the minerals. According to my invention I determine the total of all the water soluble minerals simply and quickly by a single operation.

One method of determining the content of water soluble minerals is to mix thoroughly a known weight ci the earth sample with distilled water for a period of, say, five minutes. The water is then illtered from the sample and evaporated. The residue which is the water soluble mineral content is then weighed. Also the above-mentioned ltered water may be placed in a tube of known dimensions and its ability to adsorb electromagnetic waves may be determined. 'Ihe absorption will be approximately proportionai to the quantity of contained soluble minerals.

onizable minerals A large majority of the minerals in solution, as described above, will dissociate into ions in solution. It is therefore possible by determining the ionization values in such solutions to obtain values which reflect soluble mineral values contained in such samples. The soil may be analyzed for the different minerals separately. but this is a tedious and diflicult process for ,some of the minerals. According to my method, the total of all the ionizable minerals may be determined very simply and quickly by a single operation. In view of the fact that the great majority of such minerals are ionizable, the advantages of this method are apparent.

An apparatus for determining the content is disclosed in Fig. 3 of the drawing. I provide a glass tube i having two electrodes 2 and 3 sealed in the walls of the tube. The earth sample is placed in the bottom of the tube and the tubeis then filled with/'distilled water to a predetermined helght 5. Conductors 6 and l are connected to an alternating current bridge comprising two fixed resistances li and 9, a variable rel sistance i0, a detecting device such as a telephone receiver ii and a source of alternating current l2. If the sample is very nne, it may be necessary to allow it to stand for some time in order that the fine particles may settle. Also the sample may be separated from the water by ltration and the ionic content ci the water may be determined in the above manner. Further, the electrodes may be immersed in the sample.

In the apparatus shown, the telephone receiver li is placed to the ears and then the variable re slstance i0 is actuated until the alternating current buzz disappears. The variable resistance l@ is calibrated in megohms and the reading of. the variable resistance i0 will then give the value to be recorded for the particular sample. The reciprocal of this value will be the conductivity in mlcrohms and may be plotted as indicated above to determine the location of the buried accumulation.

The plotting oi the indications of the samples tested is carried out in the manner indicated in Figs. 1 and 2, but the values will be represented in terms of conductivity in microhms.

Inorganic colloidal materials or minerals In those cases where it is desired to measure the inorganic colloidal materials or minerals which are insoluble the following procedure is employed: v

A one gram sample of soil, which has previously been dried overnight at 60 C., ground and sieved to pass a 60 mesh screen, is stirred with a high speed stirrer (3000 R. P. M.) for one minute with cc. of 0.2% malachite green dye and allowed to stand overnight in a glass stoppered bottle in a constant temperature bath of 25 C. The next day the liquid is decanted into a centrifuge tube and centrifuged for ten minutes in a clinical centrifuge (1800 R. P. M.) in order to remove suspended soil particles. The color of the liquid is then compared with suitable color standards of the same dye using a colorimeter and the concentration of the dye determined. The amount of dye adsorbed by the soil is then computed.

In cases where all of the dye is adsorbed by a sample of soil the experiment is repeated using a smaller amount of soil.

I have now described my method both generally and specically. Naturally some changes in the procedure may be eiected without departing from the invention. For example, other quantitative and qualitative tests for inorganic values may be employed, and I do not wish to be limited to the disclosure given except within the scope dened by the claims which follow.

I claim:

1. A method of geochemical exploration for subterranean petroliferous deposits which comprises collecting samples of soil at spaced points in the area to be investigated at a depth several feet below the surface, said depth being sufficient to eliminate the effect of surface conditions, determining the sulphate content of the respective samples, and determining the location of petroliferous deposits by selecting the significant high sulphate values and correlating the same with reference to the locations at which samples yielding such significant high values were obtained.

2. A method according to claim -1. in which the samples are laterally spaced from each other and are collected at a depthcof about ten feet.

3. A method of geochemical exploration for subterranean petroliferous deposits which comprises collectlng samples of soil at spaced points in the area to be investigated at a depth several feet below the surface, said depth being sumcient to eliminate the effect of surface conditions, determining the total water soluble content of the respective samples, and determining the location of petroliferous deposits by selecting the significant high total water soluble values and correlating the same with reference to the locations at which samples yielding such significant high values were obtained.

4. A method according to claim 3 in which the samples are laterally spaced from each other and are collected at a depth of about ten feet.

EUGENE McDERMO'I'I'. 

